Echo canceller ensuring further reduction in residual echo

ABSTRACT

An echo canceller includes: an echo cancellation part canceling echo included in a near-end input signal; a noise suppression part employing a low-pass filter having a variable cut-off frequency adaptive to power of an input signal, the noise suppression part removing a residual echo component from an output of the echo canceller part; a threshold generation part generating a variable threshold varying in accordance with a power level of a far-end input signal; a determination part making a determination based on a comparison between a power level of the output of the echo cancellation part and the variable threshold; and a processing part performing noise suppression using the low-pass filter on the output of the echo cancellation part when the determination part determines that the power level of the output of the echo cancellation part is lower than or equal to the variable threshold.

TECHNICAL FIELD

[0001] The present invention relates to echo canceller technology forhowling prevention and echo control, which technology is required inhands-free telephones.

BACKGROUND ART

[0002] The function means of an echo canceller is formed mainly of atransversal filter and a coefficient modifier part that successivelyupdates the coefficient of the filter. Generally, the normalized leastmean squares (NLMS) algorithm, which is excellent in stability andconvergence, relatively simple in terms of operations, and actuallyrealized as a device using an LSI, may be employed as a method ofestimating, or updating, the filter coefficient {hi} of the transversalfilter.

[0003] Further, in order to realize full duplex hands-freecommunication, the echo canceller requires, as its important functions,an echo canceller protection function and a nonlinear processing (centerclipper) function. The echo canceller protection function preventserrors from being caused by the speech of a near-end talker in updatingthe tap coefficient (filter coefficient) at the time of two-waysimultaneous communication (double talk). The nonlinear processingfunction unconditionally suppresses a low echo signal to a zero value inorder to increase the echo suppression effect when the echo level islow.

[0004] It is difficult to cancel echoes completely in a conventionalcommon echo canceller that updates the filter coefficient (tapcoefficient) of the transversal filter using the NLMS algorithm. This isbecause the conventional echo canceller treats a speech signal as itsmain target while assuming an uncorrelated signal, and cannot alwayssecure a sufficient tap length of the transversal filter with respect tothe reverberation time of a room. Accordingly, the above-describednonlinear processing is often employed.

[0005] The nonlinear processing aims to reduce residual echo by adding aso-called voice switch that turns ON or OFF depending on a signal level.Japanese Laid-Open Patent Application No. 4-150127 discloses such atechnique.

[0006] Further, Japanese Laid-Open Patent Application No. 10-285083discloses a device that includes means for varying a clip level andchanging transmitting attenuation depending on the condition oftransmission.

[0007] These voice switching methods can reduce the residual echo, buthave difficulty in dealing with the condition of double talk, so thatthere occurs the problem characteristic of the voice switch that theleading part of the speech of a near-end talker is cut off. In order tosolve the above-described problem and ensure further reduction in theresidual echo, a technology disclosed in Japanese Laid-Open PatentApplication No. 9-162787 has been proposed.

[0008] Japanese Laid-Open Patent Application No. 9-162787 discloses aconfiguration that employs a low-pass filter setting a low cut-offfrequency when signal power is low and setting a high cut-off frequencywhen signal power is high. The residual echo of an echo-canceled signalincludes a high-frequency component. Offensive noise can be reduced bysuppressing the high-frequency component. The low-pass filter isemployed as means for suppressing the high-frequency noise component.

[0009] Further, noise suppression means includes power calculation meansfor calculating the power level of an input signal supplied from an echocancellation part, comparison means for comparing the power level of thesignal calculated by the power calculation means with a predeterminedthreshold for noise determination, and noise suppression means forperforming noise suppression on the signal by the low-pass filter whenthe comparison result shows that the power level of the signal is lowerthan or equal to the predetermined threshold.

[0010] The low-pass filter of the noise suppression means varies thecut-off frequency so that the cut-off frequency is low when the signalpower is low as residual echo and the cut-off frequency is high when thesignal power is high as the speech signal of a near-end talker.Specifically, the low-pass filter of the noise suppression means isrealized by moving average processing, and the variation in the cut-offfrequency of the low-pass filter is realized by a variation in themoving average interval length of the moving average processing.Thereby, the low-pass filter (LPF) with a variable cut-off frequency ofa simple configuration can be realized.

[0011] The power calculation means obtains the power of the signalexpressed in the form of the exponent of 2^(n) by digital processing.The comparison means obtains the number of moving average intervallength bits m by subtracting the power level in the form of the exponentof 2^(n) from the noise determination threshold in the form of theexponent of 2^(n). The noise suppression means performs, by digitalprocessing, moving average processing with respect to an interval lengthdetermined by the obtained number of moving average interval length bitsm when m is not negative.

[0012] This technology, however, contains the following problems. Thatis, the noise suppression means compares the output power of the echocancellation part with the predetermined determination threshold. Thedetermination threshold can be determined with no problem when theamount of residual echo or a near-end input power value can be estimatedin advance. In the case of a great environmental variation, however, itis difficult to distinguish between the residual echo and the speech ofthe near-end talker, thus making it difficult to determine thedetermination threshold.

[0013] In the case of a fixed threshold for noise determination, if thenoise determination threshold is fixed to a high value, any signal thatis smaller than the set threshold goes through the noise (residual echo)suppression means that is the LPF with a variable cut-off frequency. Atthis time, the lower the input near-end power, the lower the cut-offfrequency of the LPF. Therefore, if the threshold setting lacksdeliberation, low-power fricative consonants may be cut by bandwidthrestriction.

[0014] On the other hand, if the determination threshold is fixed to alow value, the function of suppressing residual echo does not work. Thisis because the residual echo temporarily increases at the early learningstage of the echo canceller or at the time of echo path change. Whenfar-end input speech increases, it is natural that echo should increasein amount. However, signal distortion may occur depending on thecharacteristics of a loudspeaker on the near-end side, so that the echopath characteristic becomes nonlinear, thus increasing the residualecho. The residual echo power exceeds the threshold in such a case, thusresulting in the problem that the residual echo is transmitted to thefar-end side without going through the noise suppression part.

[0015] In order to cope with this problem, the determination thresholdmay be set to a high value so that such residual echo goes through thenoise suppression part. This, however, causes another problem of theloss of the low-power signal of the near-end talker. Thus, the residualecho and the speech of the near-end talker cannot be distinguished fromeach other by simple power comparison, so that the noise suppressionmeans does not function properly.

[0016] This problem is caused in a system where an echo coming out froma speaker to be input to a microphone has approximately the samemagnitude as that of the speech of the near-end talker. For instance,this problem is caused in a handy phone system that requires aloudspeaker and a microphone to be arranged relatively close to eachother or in an on-vehicle hands-free telephone system that outputs soundat high volume from a loudspeaker and requires a microphone to bearranged at a distance from the mouth of a speaker.

[0017] With respect to the noise suppression means using the LPF with avariable cut-off frequency, Japanese Laid-Open Patent Application No.9-275367 discloses another technology.

[0018] Japanese Laid-Open Patent Application No. 9-275367 discloses anecho canceller device has the functions of (a) detecting the directionof communication depending on the condition of communication from aninput signal from the far end (the speech of a far-end talker) and aninput signal from the near end (the combination of the speech of anear-end talker and the echo sound of the speech of the far-end talker)and (b) gradually changing the cut-off frequency of an LPF for residualecho suppression every time the direction of communication changes.

[0019] Actually, however, it is difficult to detect the direction ofcommunication under a noisy environment or in the case where thedirection of communication changes frequently, so that the residual echois transmitted to the far end if the cut-off frequency is delayed inchanging with respect to a change in the direction of communication.

DISCLOSURE OF THE INVENTION

[0020] Accordingly, it is a general object of the present invention toprovide an echo canceller in which the above-described disadvantages areeliminated.

[0021] A more specific object of the present invention is to provide anecho canceller that includes a noise determination function that isadaptive to the speech of a far-end talker so as to ensure furtherreduction in residual echo even if the residual echo increases at theearly learning stage of the echo canceller or at the time of echo pathchange.

[0022] The above objects of the present invention are achieved by anecho canceller including: an echo cancellation part canceling echoincluded in a near-end input signal to the echo canceller, the near-endinput signal including an echo component of a far-end input signal tothe echo canceller; a noise suppression part employing a low-pass filterhaving a variable cut-off frequency adaptive to power of an inputsignal, the noise suppression part removing a residual echo componentfrom an output of the echo canceller part; a threshold generation partgenerating a variable threshold varying in accordance with a power levelof the far-end input signal; a determination part making a determinationbased on a comparison between a power level of the output of the echocancellation part and the variable threshold; and a processing partperforming noise suppression using the low-pass filter on the output ofthe echo cancellation part when the determination part determines thatthe power level of the output of the echo cancellation part is lowerthan or equal to the variable threshold.

[0023] The above objects of the present invention are also achieved byan echo canceller including: an echo cancellation part canceling echoincluded in a near-end input signal to the echo canceller, the near-endinput signal including an echo component of a far-end input signal tothe echo canceller; a noise suppression part employing a low-pass filterhaving a variable cut-off frequency adaptive to power of an inputsignal, the noise suppression part removing a residual echo componentfrom an output of the echo canceller part; a first calculation partcalculating a power level of the output of the echo canceller part; asecond calculation part calculating a power level of the far-end inputsignal; a determination part making a determination based on acomparison between the power level of the output of the echocancellation part and a variable threshold varying in accordance withthe power level of the far-end input signal; and a processing partperforming noise suppression using the low-pass filter on the output ofthe echo cancellation part when the determination part determines thatthe power level of the output of the echo cancellation part is lowerthan or equal to the variable threshold.

[0024] According to the present invention, it is considered that littleor no echo exists when there is no input of far-end speech or the powerof input far-end speech is low. Therefore, the threshold for noisedetermination can be set to a low value. In the case of the conventionalfixed threshold for noise determination, any signal smaller than a setthreshold for noise determination is supplied to a noise (residual echo)suppression part that is a low-pass filter (LPF) with a variable cut-offfrequency, so that the cut-off frequency of the LPF becomes lower as thepower of a near-end input is decreased. Therefore, if the thresholdsetting lacks deliberation, low-power fricative consonants may be cut bybandwidth restriction. According to the above-described echo cancellersof the present invention, however, the threshold for noise determinationcan be varied in accordance with the power of the far-end speech, sothat the above-described disadvantage can be eliminated.

[0025] On the other hand, it is rarely the case that a high-power signalis input continuously from the far-end in a normal conversation.However, when the far-end input speech increases in power, it is naturalthat echo should increase. Further, signal distortion may occurdepending on the characteristics of a loudspeaker on the near-end side,so that the echo path characteristic becomes nonlinear, thus increasingresidual echo. Moreover, normally, the learning function of the echocanceller is suspended at the time of double talk. Accordingly,immediately after a double talk interval, not to mention at the earlylearning stage of the echo canceller, the echo path characteristics maychange greatly so as to increase the residual echo. In such a case, thethreshold for noise determination is increased in accordance with thefar-end power so as to prevent the residual echo from exceeding thethreshold. Thereby, the LPF is allowed to perform bandwidth cutting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings, in which:

[0027]FIG. 1 is a diagram showing the configuration of an echo cancelleraccording to an embodiment of the present invention;

[0028]FIG. 2 is a diagram showing the configuration of a common noisesuppression part;

[0029]FIG. 3 is a diagram showing the configuration of a noisesuppression part according to the embodiment of the present invention;

[0030]FIG. 4 is a graph for illustrating a limiting operation method;and

[0031]FIG. 5 is a graph for illustrating another limiting operationmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

[0032] A description will now be given, with reference to theaccompanying drawings, of an embodiment of the present invention.

[0033]FIG. 1 is a diagram showing the configuration of an echo cancelleraccording to the embodiment of the present invention. The echo cancellerof FIG. 1 includes an echo cancellation part 1 and a noise suppressionpart 2. According to this configuration, a signal x_(i) output from theecho cancellation part 1 is input to the noise suppression part 2.

[0034] The echo cancellation part 1 has the same function as theconventional echo canceller. That is, the echo cancellation part 1 hasthe function of canceling echo by first generating a pseudo echo bysupplying a reception signal to a filter simulating an echo path andthen applying a phase-inverted pseudo echo to an input signal. The echocancellation part 1 further includes the double-talk function and thehowling detection function in order to realize the hands-freecommunication function.

[0035] The noise suppression part 2 is formed of a low-pass filter thatsets a low cut-off frequency when the power of an input signal is lowand sets a high cut-off frequency when the power of the input signal ishigh. According to the echo canceller of FIG. 1, the output of the echocancellation part 1 is supplied to the noise suppression part 2 andthereafter, is transmitted as a transmission signal.

[0036]FIG. 2 is a diagram showing a common configuration of the noisesuppression part 2. According to the configuration of the noisesuppression part 2 of FIG. 2, first, a power calculation part 11 and apower exponent extraction part 12 calculate the power level of theoutput signal x_(i) of the echo cancellation part 1. After the powerlevel of the signal x_(i) is thus calculated, it is determined, bycomparing the power level of the signal x_(i) with a threshold ths fornoise determination, whether the signal x_(i) is a high-power signalsuch as the speech of a near-end talker or a low-power signal such asresidual echo or background noise.

[0037] According to the configuration of FIG. 2, the noise suppressionpart 2 includes the power calculation part 11, the power exponentextraction part 12, a subtraction part 13, a sign determination part 14,and a noise suppression processing part 15. The power calculation part11 calculates the power of the output signal x_(i) of the echocancellation part 1, that is, the power calculation part 11 converts theoutput signal x_(i) into power. The power exponent extraction part 12expresses the power calculated by the power calculation part 11 in theform of 2^(k), and extracts the value s of its exponent. The subtractionpart 13 obtains the number of moving average interval length bits m bysubtracting the exponent value s of the power in the form of 2^(k) fromthe noise determination threshold ths, and determines the moving averageinterval length to be 2^(m). The sign determination part determineswhether the number of moving average interval length bits m is negativeor not. The noise suppression processing part 15 performs noisesuppression on the signal x_(i).

[0038] The noise suppression part 2 further includes an output selectionpart 16. When the number of moving average interval length bits m isnegative, the output selection part 16 determines that the level of theoutput signal x_(i) of the echo cancellation part 1 (that is, the signalinput to the noise suppression part 2) is higher than the noisethreshold ths and that the signal x_(i) is the speech signal of thenear-end talker to be transmitted. Therefore, the output selection part16 outputs the as-input signal x_(i) as the transmission signal withoutthe signal x_(i) being subjected to the noise suppression. On the otherhand, when the number of moving average interval length bits m is notnegative, the output selection part 16 outputs the signal x_(i) that hasbeen subjected to the noise suppression in the noise suppressionprocessing part 15 as the transmission signal.

[0039] According to the configuration of FIG. 2, the function of alow-pass filter (LPF) in the noise suppression processing part 15 isrealized by moving average processing, and variations in the cut-offfrequency of the LPF are realized by variations in the moving averageinterval length of the moving average processing. Thereby, the LPF witha variable cut-off frequency of a simple configuration is realized. Thatis, letting x_(i), n, and X_(i) be the i^(th) sample of the inputsignal, the moving average interval length, and an approximate totalvalue for the moving average interval, an approximate moving averagevalue A_(i) is obtained by the following equations:

m=ths−s

n=2^(m)

A _(i) =X _(i) /n

X _(i)=(n−1)/n×X _(i−1) +x _(i)

[0040] This shows that an interval for the moving average processingbecomes longer as the power of the signal becomes lower, or m of thesignal becomes greater. Consequently, the LPF equivalent to a low-passfilter with a low cut-off frequency can be realized.

[0041]FIG. 3 is a diagram showing the configuration of the noisesuppression part 2 according to the embodiment of the present invention.In the configuration of FIG. 2, the noise determination threshold ths isprestored in the form of 2^(k) in a memory, for instance. If the noisedetermination threshold ths is fixed, however, the above-describedproblem occurs in an actual usage environment. Therefore, according tothe present invention, a variable noise determination threshold is setusing the input power of a far-end talker.

[0042] That is, the noise suppression part 2 according to the presentinvention further includes a power calculation part 21, a power exponentextraction part 22, a subtraction part 23, a threshold limiter 24, and aaddition part 25 for a far-end input signal in addition to theconfiguration of FIG. 2.

[0043] The power of the far-end input signal is expressed in the form of2^(k), and the power exponent extraction part 22 extracts its exponentvalue r and sets r as the power level of the far-end input signal. Thesubtraction part 23 subtracts a predetermined threshold thr for far-endspeech power detection from the power exponent value r and obtains asubtraction result mr.

[0044] Next, the subtraction result mr is multiplied by a parameter αfor reflecting the far-end power to be output as a value thra. The valuethra goes through the limiter 24 so as to have its setting range limitedso that the value thra may not be set to be higher or lower thanrequired by the effect of the far-end power. Thereby, the output valuethral of the limiter 24 is obtained.

[0045] This limiting operation may be performed by clipping as shown inFIG. 4 or by using the following equations as shown in FIG. 5:

mr=r−thr

M=mr×α${thra1} = {{th1} - \frac{\beta}{\left( {M + {\beta/{thl}}} \right)}}$

 thl=thl+(M≧ 0)

thl=thl−(M<0)

[0046] where thl+ is a positive limiter representing the upper limit,thl− is a negative limiter representing the lower limit, and β is acoefficient that determines a curve. As β becomes larger, the curvebecomes smoother.

[0047] Next, the addition part 25 adds the output value thral of thelimiter 24 to the fixed noise determination threshold ths, therebyobtaining a variable noise determination threshold thsr that is adaptiveto the far-end power as shown in the following equation:

thsr=ths+thral

[0048] Thereafter, the noise suppression part 2 of FIG. 3 performs thesame operation as in the configuration of FIG. 2.

[0049] The noise suppression part 2 of the present invention comparesthe power of the far-end signal and the output power of the echocancellation part 1, using the calculated variable noise determinationthreshold thsr. As a result, if the power level of the far-end signal isso high that residual echo is increased, the noise determinationthreshold thsr is increased so that the residual echo may be cut by theLPF. Meanwhile, if the power of the far-end signal is so low that theresidual echo is reduced, the determination threshold thsr is decreasedso as to prevent the LPF from working on the near-end signal. Bylimiting the range of the threshold thsr through the threshold limiter24, the LPF is prevented from cutting off the near-end signal more thannecessary at the time of double talk.

[0050] Thus, according to the present invention, the noise determinationthreshold is varied using the input power of the far-end talker.Therefore, the residual echo component can be cut with more accuracy, sothat its effect on a signal to be transmitted, such as the speech of thenear-end talker, can be minimized. Thereby, a transmission signal thatis so excellent as to be easy to hear can be obtained.

[0051] Further, this function can be realized with a simpleconfiguration, so that the amount of calculation can be reduced inrealizing this function in a DSP.

[0052] The present invention is not limited to the specificallydisclosed embodiment, but the variations and modifications may be madewithout departing the scope of the present invention.

[0053] The present application is based on Japanese priority applicationNo. 2001-355299 filed on Nov. 20, 2001, the entire contents of which arehereby incorporated by reference.

1. An echo canceller comprising: an echo cancellation part cancelingecho included in a near-end input signal to the echo canceller, thenear-end input signal including an echo component of a far-end inputsignal to the echo canceller; a noise suppression part employing alow-pass filter having a variable cut-off frequency adaptive to power ofan input signal, the noise suppression part removing a residual echocomponent from an output of said echo canceller part; a thresholdgeneration part generating a variable threshold varying in accordancewith a power level of the far-end input signal; a determination partmaking a determination based on a comparison-between a power level ofthe output of said echo cancellation part and the variable threshold;and a processing part performing noise suppression using the low-passfilter on the output of said echo cancellation part when saiddetermination part determines that the power level of the output of saidecho cancellation part is lower than or equal to the variable threshold.2. The echo canceller as claimed in claim 1, wherein said echocancellation part cancels the echo included in the near-end input signalby generating a pseudo echo and applying a phase-inverted pseudo echo tothe near-end input signal, the echo cancellation part generating thepseudo echo by supplying the far-end input signal to a filter simulatingan echo path.
 3. The echo canceller as claimed in claim 1, wherein thecut-off frequency of the low-pass filter of said noise suppression partis decreased when the power of the input signal is low and is increasedwhen the power of the input signal is high.
 4. The echo canceller asclaimed in claim 1, further comprising a threshold limiter setting anupper limit and a lower limit to an effect of power of the far-end inputsignal so as to set an upper limit and a lower limit to the variablethreshold.
 5. The echo canceller as claimed in claim 1, furthercomprising: a first calculation part calculating the power level of theoutput of said echo canceller part; and a second calculation partcalculating the power level of the far-end input signal.
 6. The echocanceller as claimed in claim 5, further comprising a threshold limitersetting an upper limit and a lower limit to an effect of power of thefar-end input signal so as to set an upper limit and a lower limit tothe variable threshold.
 7. The echo canceller as claimed in claim 6,wherein said threshold generation part generates the variable thresholdbased on an output of said threshold limiter.
 8. The echo canceller asclaimed in claim 6, wherein said threshold generation part generates thevariable threshold based on a difference between the power level of thefar-end input signal and a predetermined threshold.
 9. The echocanceller as claimed in claim 6, wherein said noise suppression partcomprises said threshold generation part, said determination part, saidprocessing part, said first calculation part, said second calculationpart, and said threshold limiter.
 10. An echo canceller comprising: anecho cancellation part canceling echo included in a near-end inputsignal to the echo canceller, the near-end input signal including anecho component of a far-end input signal to the echo canceller; a noisesuppression part employing a low-pass filter having a variable cut-offfrequency adaptive to power of an input signal, the noise suppressionpart removing a residual echo component from an output of said echocanceller part; a first calculation part calculating a power level ofthe output of said echo canceller part; a second calculation partcalculating a power level of the far-end input signal; a determinationpart making a determination based on a comparison between the powerlevel of the output of said echo cancellation part and a variablethreshold varying in accordance with the power level of the far-endinput signal; and a processing part performing noise suppression usingthe low-pass filter on the output of said echo cancellation part whensaid determination part determines that the power level of the output ofsaid echo cancellation part is lower than or equal to the variablethreshold.